Metabolic acidosis is the most common disorder of acid-base balance, characterized by decreased primary HCO3- (<21 mmol/L) and decreased pH (<7.35) due to increased extracellular fluid H+ or loss of HCO3-. In the clinical judgment of metabolic acidosis, the anion gap (AG) has an important clinical value. According to different AG values, it can be divided into high AG normal chloride type and normal AG high chloride type metabolic acidosis. High AG normal chloride metabolic acidosis includes: lactic acidosis, ketoacidosis, metabolic acidosis due to drugs or toxic substances, and uremic metabolic acidosis. Normal AG hyperchlorotic metabolic acidosis is mainly caused by loss of HCO3- from the kidneys or outside the kidneys, or reduced renal tubular H secretion but relatively normal glomerular filtration function. Either HCO3- loss or reduced tubular H secretion alone results in too little HCO3-, while there is generally no accumulation of other organic anions in the blood, so Cl- levels rise accordingly, mostly in normal AG hyperchloremic acidosis.